1. Field of the Invention
The present invention relates to a method for producing a member (e.g., a fixing roller, a fixing belt, a pressing roller, or a pressing belt) used in a fixing unit of office automation (OA) equipment such as a copier; and such a fixing-unit member.
2. Description of the Related Art
In general, an image-forming apparatus such as an electrophotographic copier, a facsimile, or a laser-beam printer forms an image by, for example, a charging step of uniformly charging a photosensitive drum; an exposure step of exposing an image to form an electrostatic latent image on the photosensitive drum; a development step of making toner adhere to the electrostatic latent image to form a toner image; a transfer step of transferring the toner image on the photosensitive drum onto a receiver such as a paper sheet or a synthetic-resin sheet; a fixing step of fixing the unfixed toner image on the receiver; and a cleaning step of removing residual toner on the photosensitive drum.
In these steps, various roller members and/or belt members are used such as a charging roller/belt, a transfer roller/belt, a development roller/belt, a fixing roller/belt, and a pressing roller/belt. These members are required to have functions according to their usage.
For example, in the fixing step, an unfixed toner image on the receiver is generally fixed on the receiver by being heated and pressed. In an existing fixing unit (fixing section) that fixes toner images by heating and pressing, a cylindrical fixing roller and a cylindrical pressing roller are disposed so as to face each other; a receiver having an unfixed toner image thereon is passed through the nip between these rollers so that the unfixed toner image is heated and pressed to thereby be fixed on the receiver. The fixing roller has a heater such as an electric heater therein and controls the surface temperature of the fixing roller with the heater. However, the time it takes the surface temperature of such a fixing roller to be increased to the fixing temperature with such an internal heater is long. Thus, the waiting time from the turning-on of power to the operable state is long, which is problematic.
To address this problem, a fixing unit in which a heater is disposed so as to face a pressing roller with a rotatably supported thin fixing belt therebetween has been developed in recent years. By passing a receiver having an unfixed toner image thereon through the nip between the thin fixing belt and the pressing roller, heat provided by the heater substantially directly propagates to the unfixed toner image through the thin fixing belt. Thus, a fixed toner image can be formed after a very short waiting time from the turning-on of power.
In recent years, an increase in print (printing and copying) speed, output of full-color images, and a decrease in power consumption have been demanded for image-forming apparatuses. To increase the print speed, the heating efficiency of the fixing unit needs to be increased to thereby rapidly fix unfixed toner images on receivers.
To form a full-color image, in the development step, development is performed sequentially with color toners such as cyan toner, magenta toner, yellow toner, and black toner to form color-toner images; and, in the transfer step, the color-toner images are transferred onto a receiver so as to be sequentially stacked on top of one another. In addition, in the fixing step, such a multicolor unfixed toner image, which is thicker than a single-color unfixed toner image, is heated and pressed in such a manner that the toners are melted without causing mixing of colors to thereby fix the unfixed toner image on a receiver. For this reason, the fixing unit needs to have a high heating efficiency.
In an image-forming apparatus, the fixing unit consumes a large amount of energy. Thus, to achieve energy saving, the power used for heating in the fixing unit is desirably decreased or the heating efficiency during the fixing is desirably increased. An increase in the heating efficiency in the fixing unit can lead to saving of electric power.
In various members of such an image-forming apparatus, a fixing roller/belt and a pressing roller/belt of the fixing unit used in the fixing step are required to (1) have surfaces from which toner is readily released so that unfixed toner on receivers does not adhere to the surfaces; (2) have an excellent thermal conductivity such that unfixed toner images can be efficiently fixed on receivers; and (3) have excellent durability so as to provide stable functions for a long period of time. In the specification, a fixing roller, a fixing belt, a pressing roller, and a pressing belt are collectively referred to as a “fixing-unit member”. Note that such a fixing belt may be used in the form of an endless belt or a sleeve and may be incorporated as, for example, a fixing sleeve into a fixing unit.
To satisfy such requirements, a fixing-unit member is used in which a layer of a fluorocarbon resin, which is a material excellent in terms of toner releasability, heat resistance, and durability, is formed on the surface of a rubber roller constituted by a base and a rubber elastic layer.
Such a fluorocarbon-resin layer is formed of one or more fluorocarbon resins such as a tetrafluoroethylene-hexafluoropropylene copolymer (FEP), polytetrafluoroethylene (PTFE), an ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), an ethylene-chlorotrifluoroethylene copolymer (ECTFE), and polyvinylidene fluoride. In particular, a tetrafluoroethylene-perfluoro(alkyl vinyl ether) copolymer (PFA) is preferably used.
PFAs have sufficiently high heat resistance, melt flowability, and extrusion formability and fluorocarbon-resin layers formed of PFAs are excellent in terms of toner releasability, surface smoothness, and the like. PFAs generally have a lower melting point (about 280° C. to 320° C.) than PTFE, which is also a fluorocarbon resin. Thus, compared with a PTFE layer, a PFA layer can be formed on the rubber elastic layer of a rubber roller at a low temperature and hence the rubber elastic layer does not become deteriorated.
For example, a fixing-unit member having a fluorocarbon-resin layer on its surface can be produced as follows. A fixing roller having a fluorocarbon-resin layer may be produced by a method in which a fluorocarbon-resin coating solution is applied to the inner surface of a mold and baked to be turned into a hollow cylindrical cured membrane; a roller shaft is then inserted along the longitudinal axis of the mold and liquid rubber is injected between the cured membrane and the roller shaft; and the rubber is thermally vulcanized. Alternatively, a fluorocarbon-resin layer may be formed on the surface of a roller including a metal shaft and a rubber elastic layer and the like formed around the metal shaft, by applying a fluorocarbon-resin coating solution to the surface of the roller and baking the applied solution.
However, in such a method using a mold, when a fixing roller is released from the mold, there are cases where a portion of a fluorocarbon-resin layer comes off or wrinkles are generated in the fluorocarbon-resin layer. Use of a mold increases the production cost, which is also problematic. In the method of applying a fluorocarbon-resin coating solution, the baking temperature of the fluorocarbon resin is higher than the upper temperature limit of a rubber elastic layer and hence there are cases where the rubber elastic layer becomes deteriorated.
Japanese Unexamined Patent Application Publication No. 64-1534 proposes a method in which a thermally flowable fluorocarbon-resin thermally shrinkable tube is fixed by being shrunken onto a cylindrical article having been coated with a primer, and the tube and the article are then thermally fused together at a temperature equal to or higher than the melting point of the fluorocarbon resin. However, in this method, the fusion temperature is 330° C. to 400° C. and hence the rubber elastic layer may become deteriorated.
Japanese Examined Patent Application Publication No. 47-20747 and the like propose a method for producing a fixing roller in which a thermally shrinkable fluorocarbon-resin tube is thermally shrunken to be fixed onto a rubber roller. However, this method requires an extra production step of treating the inner surface of the thermally shrinkable fluorocarbon-resin tube, which complicates the production. In addition, the adhesion strength between the tube and the rubber roller is unstable and the durability of the adhesion between the tube and the rubber roller is low, which are problematic.
International Publication No. 2008/126915 describes a method for producing a fixing roller or a fixing belt by sequentially forming an elastic layer and a surface layer on a base in which a PFA-containing adhesive is applied to the outer circumferential surface of the elastic layer; this elastic layer is sheathed with a thermally shrinkable PFA tube having an inner circumferential diameter slightly larger than the outer circumferential diameter of the elastic layer; and the PFA tube and the adhesive are then heated at a temperature equal to or higher than the melting points of the PFA tube and the PFA contained in the adhesive such that the PFA tube is 3% to 20% thermally shrunken and the outer circumferential surface of the elastic layer and the inner circumferential surface of the PFA tube are bonded together through the adhesive to thereby form a PFA layer serving as a surface layer on the elastic layer.
Japanese Unexamined Patent Application Publication No. 2004-276290 describes a method for producing a fluorocarbon-resin-coated roller in which an adhesive having a low viscosity is applied to the inner circumferential surface of a fluorocarbon-resin tube having an inner diameter smaller than the outer diameter of a roller base (corresponding to a rubber roller) and/or the outer circumferential surface of the roller base; and then the roller base is sheathed with the fluorocarbon-resin tube while an end of the fluorocarbon-resin tube is diametrically expanded and, during this sheathing, the adhesive functions as a lubricant. This publication states that, in this method, since the roller base is sheathed with the fluorocarbon-resin tube having an inner diameter smaller than the outer diameter of the roller base while an end of the fluorocarbon-resin tube is diametrically expanded, the fluorocarbon-resin tube does not become wrinkled and, in addition, the fluorocarbon-resin tube and the roller base can be tightly bonded together.